The present invention has for its object a process and a device for machining by electroerosion, according to which a piece is machined by means of a tool, they being separated from each other by a machining distance of a working space in which is located a machining fluid, by applying an electrical voltage between the tool and the piece to cause erosive discharges.
The processes and devices for machining by electroerosion uses tools electrodes which can be wires, tubes or electrodes having any shape. The electrode is not in physical contact with the piece; it is separated therefrom by a fluid.
Certain users of machines have observed that the duration of the life of the tools produced by electroerosion can be shorter than that of the tools produced by conventional mechanical methods.
The cause of this problem is an electrolytic phenomenon which damages the surfaces of the pieces machined by electroerosion. Electrolysis can render the machined surfaces less hard and less homogeneous than before their machining. This is particularly the case when the machined piece is of a sintered material.
Electrolysis, which takes place during machining, is an undesirable secondary effect which is produced with all the machines using water, or any other liquid or fluid whose conductivity is too high to separate the electrode from the piece.
The effects of electrolysis are proportional to the conductivity of the liquid, which is to say its content of electric charge carriers (ions and electrons). This is why, when water is used, it must be deionized so that it contains as few ions as possible. But its nature is such that even very pure water contains ions in suspension.
To machine a piece by electroerosion, it is well known that it is necessary first to apply a voltage between the electrode and the piece. It is also well known that the current discharge serving for machining does not take place immediately after application of the voltage, but that there is a certain delay which is called the delay time by those skilled in the art.
During this delay time tD, under the influence of the electric field, the ions of negative polarity present in the liquid are drawn by the piece of positive polarity and continuously bombard it.
The ions also react chemically with the piece, forming salts which in turn go into solution. In other words, the metal at the surface of the piece is subjected to metal-lurgical modifications.
To avoid this undesirable electrolysis phenomenon, there are used fluids which contain only few ions. The fluid therefore should not take part in an electrochemical process. There has thus been used as the dielectric fluid, mineral oil.
However, the absence of electrically charged particles in the oil is not assured, particularly during machining in which the oil contains many eroded particles created by electroerosion, as well as molecules from the cracking of the oil caused by the discharges. Such a counter-measure can accordingly only decrease the problem of electrolysis.
Moreover, in the case of machining with wire electrodes, the solution consisting in using oils as the dielectric is an alternative whose principal drawback is a great reduction of the speed of machining.
In the case of machining with massive electrodes, oil does not eliminate the risk of the appearance of electrically charged machining sludge which agglomerates and clings to one of the electrodes.
It is also possible to maintain the conductivity of the water at as low a level as possible. This solution only decreases the problem because it has been determined that it is not possible to obtain water which contains no ions.
There have also been carried out certain machining steps, in particular finishing passes, by reversing the polarity applied between the electrode and the piece. In this case, the electrode has a positive electrical charge relative to the piece. Thus, the electrochemical phenomenon to be avoided on the piece, is transferred to the electrode. The drawback of this approach is that the finishing operations become slower, without at the same time completely avoiding the electrolysis phenomenon.
There has also been proposed in the patents CH 536166 and U.S. Pat. No. 4,347,425, using a generator comprising two voltage sources, of opposite polarities. These sources can be engaged such that the electrode and the piece have, during the delay time, a polarity opposite to that which they have during establishment of the principal current discharge. The damaging effects of electrolysis can thus be reduced. But these generators are not able to prevent electrolysis from happening.
Other solutions to guard against electrolysis consist in starting the voltage sources, continued and applied intermittently as in EP 0 545 156 or alternately and at high frequency as in U.S. Pat. No. 4,447,696 or U.S. Pat. No. 5,698,115 during a pause time separating two successive current discharges between the electrode and the piece. These solutions have the drawback of not avoiding electrolysis when it is particularly great, which is to say during the delay time tD when the generally high triggering voltage is applied.
The present invention has for its object to overcome these drawbacks and to provide a process and device for machining by electroerosion permitting avoiding the electrolysis phenomena.
The invention is characterized to this end by the following sequence of phases during erosive discharge:
a) a delay phase during which the alternating voltage has a frequency applied between the tool and the piece such that ions present in the machining fluid follow oscillating paths substantially shorter than said machining distance separating the tool and the piece and during which an ionized passage is created between the tool and the piece by the effect of said alternating voltage,
b) a spark phase beginning with the detection of a current flowing between the tool and the piece and during which a source of continuous voltage forms and applies an erosive discharge current, and
c) a pause phase during which no voltage is applied between the tool and the piece such that the machining fluid present between the tool and the piece deionizes and the ionized path disappears;
and by the fact that this sequence of phases is repeated throughout the machining.
These characteristics permit obtaining a machining process without an electrolysis phenomenon, with easy triggering of the discharges and high machining efficiency, whilst avoiding deterioration of the piece or the tool, for example breaking of the wire, by application of excessive heat to a given portion of this tool.
Preferably, the process is characterized by the fact that the frequency of said alternating voltage is at least 100 kHz for a working space providing a machining distance of 30 to 40 microns, by the fact that this frequency is increased when said distance is decreased, and by the fact that this frequency is substantially 10 MHz for a distance substantially equal to 1 micron.
Thus the oscillating path of the ions is much shorter than the machining distance and all bombardment of the piece and the tool by the ions present in the machining liquid can be avoided.
According to a preferred modification, the frequency of the alternating voltage is variable during the delay time.
Preferably, the frequency of the alternating voltage decreases progressively within the delay phase so as to sweep a spectrum of frequencies.
These characteristics permit improving effectively the production of an ionized path and hence the triggering of the discharges.
Preferably, there is established an alternating voltage such that the duration of the negative pulses will be different from that of the positive pulses.
The mean voltage can thus be adapted across the machining gap or distance so as to guarantee the absence of any electrolysis phenomenon, while optimizing the machining conditions.
These advantages can be further improved by the fact that there is established an alternating voltage such that the amplitude of the negative pulses will be different from that of the positive pulses.
According to an interesting modification, the alternating voltage is blocked over the random polarity present at the moment of detection of a current between the tool and the piece.
These characteristics permit avoiding the extinction of a discharge triggered by a change of polarity during establishment of the erosive discharge.
Another modification of the process is characterized by the fact that the alternating voltage is blocked upon the detection of a current between the tool and the piece by the fact that said predetermined polarity is preferably that which corresponds to the polarity of the continuous voltage source.
These characteristics also permit reducing the risk of extinction while preserving measures against the electrolysis phenomena.
A particularly favorable modification of the process is characterized by the fact that the sparking phase is subdivided into a delay time corresponding to the duration of triggering of the principal discharge by the continuous voltage source, and a current time corresponding to the erosive discharge, and by the fact that the duration of said delay time is adjusted such that there is obtained for each erosive discharge at least one complete period of the alternating voltage comprising a positive pulse and a negative pulse.
These characteristics also permit refining in a controlled and precise fashion the anti-electrolytic measures.
Thus, the lag time is preferably regulated such that
Trxe2x89xa7Txe2x88x92tD
in which
TR is the duration of the time lag
T is the period of alternating voltage, and
TD is the duration of the delay phase
The present invention also relates to a device for practicing a process for machining by electroerosion according to which a piece is machined by means of a tool, they being separated from each other by a machining distance of a working space in which is located a machining fluid, by applying an electrical voltage between the tool and the piece, characterized by the fact that it comprises in combination
an alternating voltage source arranged to produce during a delay phase an alternating voltage at a frequency such that, between the tool and the piece, the ions present in the machining fluid follow an oscillating path substantially shorter than the machining distance and that an ionized path is created between the tool and the piece,
a detector adapted to detect a current between the tool and the piece,
a source of continuous voltage arranged so as to form and maintain an erosive discharge current during a sparking phase beginning with the detection of a current between the tool and the electrode, and
a control unit arranged to start said alternating and continuous voltage sources so as to obtain during erosive discharges a succession of phases comprising said delay phase, said sparking phase, and a pause phase during which no voltage is applied between the tool and the piece, such that the machining fluid between the tool and the piece deionizes and the ionized path disappears.
The device thus permits eliminating all electrolytic phenomena while permitting effective machining without thermal damage to the tool or to the piece.
The characteristics according to the present invention require that during the delay phase, the ions move only very little under the influence of the high frequency alternating electric field. The ions oscillate about an equilibrium position, but the distance over which oscillation takes place is very much less than the width of the space between the tool and the piece. Thus, no ionic bombardment is possible. On the other hand, the mobility of the electrons, several tens of thousands times lighter than the lightest ions, is such that they can follow a high frequency electrical field and thus permit triggering. The pause phase is necessary to deionize completely the dielectric of the gap, which accordingly does not preserve any memory of the position of the ionized path of the preceding discharge. It is thus avoided that two or more discharges take place at the same spot and give rise to damage to the tool or to the piece, for example breaking of the wire, by excessive application of heat to this spot.